The invention concerns an overvoltage protection circuit, and in particular, although not exclusively, an electrostatic discharge protection circuit for an integrated circuit device for use with high-voltage power supplies.
Integrated circuits are often damaged by voltage transients which can overload active elements within the device and cause permanent damage. Diodes are used in most devices for internal transient protection, as shown in FIG. 1. In FIG. 1, diodes 11 and 12 are connected to an input node 13 of an integrated active circuit 14 (e.g. an amplifier) such that in normal operation they are reverse-biased, but when the node 13 is subjected to a transient outside the normal supply voltage range, one or other of diodes 11 and 12 becomes forward-biased and protects the internal circuitry of the active circuit.
A problem arises, however, when high-voltage supplies are to be used with integrated circuits formed by processes yielding inherently low-voltage devices. Under these circumstances, the diodes can be subjected to voltage drops which cause breakdown of the semiconductor junctions forming the diodes. An example of this is the use of a 30 v supply on an, integrated circuit comprising devices having a breakdown voltage of only 15 v.
It would be desirable to provide an overvoltage protection circuit which can be used in place of a conventional diode clamp to overcome the above-mentioned drawback.